Exploring Molecular Mechanisms of Establishment of the Winter-annual Trait and Regulation of Vernalization in Arabidopsis thaliana

In flowering plants, vernalization is a finely regulated process that results in competence to flower in response to a passage of winter. Natural variation at dominant alleles of FRIGIDA ( FRI) and FLOWERING LOCUS C (FLC) determines the vernalization requirement in Arabidopsis. FRI activates FLC expression and FLC blocks flowering until plants are vernalized. In this study, ARABIDOPSIS TRITHORAX RELATED 3 (ATXR3), a H3K4 methyltransferase, is proposed to be an FLC activator via depositing active histone markers on FLC chromatin. Arabidopsis mutants with lesions in ATXR3 exhibit early flowering in the presence of FRI, consistent with decreased FLC as well as reduced H3K4me3 levels at FLC chromatin in the atxr3 mutant, which suggests that ATXR3 plays a key role as a transcriptional activator in establishing a winter-annual (i.e. vernalization-requiring) habit. To study vernalization regulation, I took advantage of a VIN3-reporter transgene and a radish VIN3 transgene to identify a cis-regulatory region consisting of the promoter and the first intron that is required for cold-dependent VIN3 induction. Also, I demonstrated rapid induction of radish VIN3 in Arabidopsis, suggesting that the more rapid vernalization response of radish involves gain or loss of cis-element(s) in VIN3. Over-expressed VIN3 is not sufficient to initiate the vernalization response without cold exposure, indicating that other cold-regulated factors are involved. I performed forward and reverse genetic screens using the defined cis-regulatory region and identified PRECOCIOUS INDUCTION OF VIN3 mutants and transcription factors which bind to VIN3. Further studies on these identified factors may help to elucidate the mechanisms of VIN3 regulation.